Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum connects to a reservoir for storing the ink. A plurality of such resistor elements forms a pattern in a printhead. This pattern is called primitive. Each of the resistor elements line up with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprises an ink-jet pen.
In operation, each resistor element connects through a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. Firing a plurality of such resistor elements, in a particular order, in a given primitive, forms alphanumeric characters, performs area-fills, and provides other print capabilities on the medium.
Ink-jet inks used in thermal ink-jet printing typically comprise a colorant and a vehicle, with the vehicle often containing water and other relatively low surface tension liquids.
The tight tolerances of the nozzles (typically 50 mm diameter) require that the ink not clog the nozzles. Furthermore, repeated firings of the resistor elements, which must withstand about 10 million firings over the life of the ink cartridge, can result in fouling of the resistor element. Kogation is the term used for this type of fouling. The ink composition must be capable of interacting with the print medium, especially paper, to penetrate it without undue spreading. Finally, the ink composition should be smear and water resistant on the paper.
Many inks possess one or more of the foregoing properties. However, few ink compositions posses all of those properties, since an improvement in one property often results in the degradation of another. Thus, inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate performance in each of the aforementioned properties.
Accordingly, investigations continue into developing ink formulations that have improved properties such as reduced kogation and reduced crusting (i.e. improved decap). Further properties sought to be improved include high edge acuity, high optical density, fast drying times, good waterfastness, and good smearfastness all without sacrificing performance in other necessary properties.
Recent advances in ink-jet inks have incorporated pigments in place of the earlier water-miscible dyes. However, pigments by definition are not water-miscible, and require the presence of a dispersing agent. Further investigations into the use of pigments have determined that surface treatment of the pigment, to form either anionic or cationic functional groups on the surface, provides a macromolecular chromophore (MMC) that is water-soluble, thus eliminating the need for a dispersing agent. However, the extent of the chemical treatment can adversely affect one of the prime properties of a pigment, namely, its waterfastness.
Accordingly, investigation continues into the use of pigments that have been treated to be water-miscible, but which retain their desired waterfastness.